1. Field of the Invention
The present invention relates to an optical delay line, which is an optical control element for use in optical fiber communication, and a manufacturing method therefor.
2. Description of the Prior Art
Nowadays, a higher speed is sought after in optical fiber communication, not only on trunk lines but also on optical subscriber lines, and studies are made on capacity expansion by use of ultra-short pulses and photonic networks performing no photoelectric conversion at repeaters. Especially at each repeater (node) of such an optical communication system, an optical delay line to generate a specific time delay for controlling optical signals is required, and an optical control device for that purpose constitutes one of the key devices. Whereas a specific way of generating an optical delay is usually to have the light pass an optical fiber loop or the like having a specific distance, the method of matching with a specific wavelength is to distinguish the wavelength with a filter and to connect it to the optical fiber loop with a coupler or the like.
On the other hand, an optical fiber diffraction grating (fiber grating) in the core of the optical fiber has a narrow band filter characteristic, and excels in stability and in the efficiency of the use of light. However, since the optical fiber diffraction grating uses a reflected spectrum, it is difficult to be made a transmission type element, and therefore has to be used in combination with an optical circulator. This makes it difficult to realize the element at a low cost.
Waveguide couplers having a configuration of using no optical circulator includes one disclosed in the Published Japanese Translation of Unexamined PCT Application No. Hei 9-505673. Its elemental configuration is shown in FIG. 1, in which reference numerals 10 and 20 denote optical fibers; 30, an input end; 40 and 50, glass blocks to which the optical fibers 10 and 20 are respectively fixed; 25, a diffraction grating arranged in the core of an optical fiber; and 45, a coupling region having a length Lc of the two fibers 10 and 20 exposed from the surfaces of the glass blocks 40 and 50.
The optical fiber 10 and the optical fiber 20 constitute a directional coupler in which the cores of the two fibers are arranged close to each other. The coupling length Lc is so set that lights of a plurality of wavelengths coming incident from the input end 30 all transfer from the optical fiber 10 to the optical fiber 20. However, the light of a Bragg wavelength (λ1=λB) from the diffraction grating 25 cannot transfer to the optical fiber 20, and is outputted as T1. Therefore, lights of other wavelengths (λ2, λ3 . . . ) than the Bragg wavelength are outputted as T2.
The waveguide coupler of the configuration described above, though able to manifest a wavelength filtering function, cannot function as an optical delay line for a specific wavelength. It involves a further problem that, though it can constitute a delay line for a specific wavelength when combined with an optical fiber loop, an optical circulator and an optical coupler would be required separately. Still another problem is that, in order to form a directional coupler after the formation of an optical fiber diffraction grating, precise and uniform machining of a large area of cladding giving rise to no thermal process is needed, but there is scarcely a practical means to meet this need.
An object of the present invention, intended to solve the problems of the prior art noted above, is to provide an optical delay line for generating a specific time delay for controlling optical signals and a manufacturing method therefor.